Especially as they had a son who was human as far as I recall from the audio? And Alex was a bit freaked out to find out that his mother was an alien. Which raises the question again as to whether Susan was Time Lord or 'just' Gallifreyan.
The Doctor wanted Alex to have an education on Gallifrey so that he could fulfil his full academic potential as a Time Lord. However, Alex saw Earth as his home and decided to stay with his mother. (AUDIO: An Earthly Child)
During this visit, the Doctor analysed Alex's DNA and discovered that Gallifreyan genes constituted only 7% of his genome. Unlike full blooded Time Lords, he was not telepathic and could not regenerate.(AUDIO: Relative Dimensions)
Comments
http://tardis.wikia.com/wiki/Alex_Campbell
Thanks, I havn't got hold of Relative Dimensions yet, I must do that sometime:)